Various methods and means have been employed to mount individual lens blanks in a blocking tool which can be detachably connected to a spindle of an abrading machine in order to generate, grind and polish a surface on the lens. Conventional procedures utilized heretofore have employed a blocking pitch or wax to adhere the lens blank to the block so that the same may be processed. Such pitches have tended to be considered more or less trade secrets in the optical industry and have varied in accordance with the type of lens and glass composition to be ground, as well as the methodology and apparatus employed in the grinding process itself. Among other things, however, conventional pitches are known to include such materials as asphalt, coal tar, pine tar, rosin, beeswax, paraffin, shellac, turpentine, etc. in different proportions. Such pitches required considerable cooking and processing whereby cooking time and/or turpentine content was varied to alter the consistency or "temper" of the pitch, particularly with respect to its flow characteristics under processing conditions. Early forms of pitch had considerable flow at temperatures of over 90-95.degree. F. Since such temperatures are typically attained in grinding processes employing fast grinding speeds and/or significant pressure at the interface of the lens blank and the grinding element itself, more advanced, higher temperature pitches were ultimately formulated. Such pitches now attain flow temperatures of 140-200.degree. F., and higher.
While higher temperature pitches tend to allow for faster processing speeds and grinding pressures, they inherently transfer some of this heat to the lens blank during the bonding step creating thermal gradients within the glass resulting in lens "springing" or warpage. This warpage is particularly noted and magnified in lens geometries having a diameter-to-center thickness ratios (i.e., aspect ratios) exceeding 10:1. In addition to the difficulties associated with the thermal gradients induced by such pitches, it has been found that as pitches having greater adherent capabilities were produced, it became increasingly difficult, if not impossible, to remove the finished lens from the block without damaging the lens itself due to the strength of the bond provided by the pitch. Attempts to lessen the tenacity of the pitch generally resulted in a substantial number of lenses parting from blocks during the abrading process causing irreparable damage to the lenses. Although these detriments could be overcome by further modifying the natural pitches, the so modified pitches tended to produce various other defects in the lenses such as staining, scratching and striping.
Initial efforts to overcome the difficulties associated with the use of natural pitches focused on synthetic thermoplastic resins and cellulose derivative compositions (Hicks U.S. Pat. No. 2,434,614), as well as combinations of a selectively soluble resinous material such as polyvinylacetate in conjunction with low melting point fusible alloys (Cox et al. U.S. Pat. No. 3,404,488). Furthermore, Dent et al. (U.S. Pat. No. 4,619,082) taught the use of molten waxes in conjunction with a lens blank holder for providing added support and alignment precision in the lens manufacturing process. Such molten waxes generally had melt temperatures in the range of 60-70.degree. C.
While these latter improvements overcame some of the difficulties with the use of natural pitches, most notably the inconsistency in adhesive strength, the environmentally undesirable solvents and solutions necessary for fully removing and cleaning the pitch from the lens piece after deblocking, as well as the staining and striping noted with the use of pitches, such improved processes still subjected the lens element to thermal gradients during the blocking and deblocking processes. Furthermore, they still have the detriment of comparatively long setting time from the time at which the lens is seated on the block element and the time to which the molten adherent is sufficiently cooled to allow processing of the lens blank element.
In view of the foregoing, there continues to be a strong need and desire for the identification and development of other adhesive materials which may be used in the blocking and deblocking of lens blanks and lenses. In this respect, room temperature-hardenable adhesives, cyanoacrylate-type adhesives and two-part adhesives have been evaluated. Of these, room temperature-hardenable adhesives typically rely upon some environmental factor, most notably moisture, to initiate and bring about cure. Because these environmental factors are not readily controllable, at least not without difficulty and costly equipment, such compositions are not suitable for use as a lens blocking adhesive as there would be very little, if any, control of cure speed and bond strength. On the other hand, while cyanoacrylate-type adhesives cure instantly, because of the instant curability there is little if any time available in which to adjust the lens blank on the block element before the adhesive has fully bonded. Consequently, if the lens blank is not positioned accurately and correctly to begin with, the lens must be removed from the block element and reseated, perhaps repeatedly, until it is seated correctly. Furthermore, this type of adhesive exhibits excessively high bonding forces requiring longer deblocking times. Finally, as with any two-part adhesive, since, by definition, they require the mixing of two components to initiate cure, (which in itself is subject to variation and consequently performance variation), once the materials are mixed they must be used within a relatively short period of time due to their limited pot life. Although the pot life can be extended, it is done so at the expense of cure speed.